1. Field of the Invention
This invention relates to a toner concentration control device for maintaining a constant toner concentration in a developer of an electrophotographic copying machine.
2. Description of the Prior Art
A developer usually employed for a dry type developing device is a mixture of toner and carrier. The toner is a fine colored resin powder. The carrier consists of particles which are treated so as to attract the toner around them with the aid of the static electricity caused by the friction between the particles and the toner. If, when such a developer is used, the developer toner density is higher than a normal or suitable value, then the amount of floating toner like smoke, not combined with the carrier, is increased. As a result, in the copy of an image, the toner is stuck to portions which should be white, that is, a phenomenon known as "fogging" occurs. On the other hand, the toner is scattered in the developing device to make the latter dirty. If, in contrast, the toner density is decreased, then the density of the reproduced image is decreased as a whole, which leads to incorrect reproduction of the original. Thus, if the toner density is not suitable, then the quality of a copied image is lowered, and not only the service life of the developer but also the service life of the entire device is decreased. These difficulties are well known in the art.
In order to maintain the device as well as the quality of a copied image satisfactorily, it is necessary to complement the amount of toner consumed in copying at required intervals in order to maintain the toner density within a suitable density range.
In order to supply the toner into the developer, means mechanically supplying a predetermined amount of toner at predetermined time intervals is, in general, employed. However, since there are a variety of originals to be copied which are different in the quantity of toner consumption, it is difficult to supplement the amount of consumed toner merely by mechanically supplying the toner. Therefore, the operator of the copying machine operates the dial provided for adjusting the amount of toner supply judging from the state of a copied image, thereby to obtain a copied image which is considered suitable in density. Even if the adjustment dial is operated by the operator, the developer density cannot immediately follow the operation because of the behavior characteristic of the developer. Accordingly, several to several tens of sheets are wasted until a copied image having a desired density is obtained. If the operating procedure of the adjustment dial is improper, then the developer density is greatly shifted from its suitable density range, and not only the copied image but also the entire developing device is adversely affected.
In order to overcome this difficulty and to obtain copied images correct in density at all times, the operator must nervously monitor the density of each copied image and operate the adjustment dial, which makes the operation of the copying machine considerably troublesome and intricate. In order to provide a copying machine which can remove such a burden from the operator, it is necessary to provide a precision toner supplying device which can weigh and supply the amount of toner which has been precisely calculated. However, in this case, the copying machine is necessarily intricate in construction and high in manufacturing cost.
In order to overcome the above-described difficulties, a variety of methods have been proposed in which the toner density is detected by electrical, magnetic or optical means, so that the amount of consumed toner is automatically supplemented to maintain the density correct. However, since the developer employed in the electrophotography special powder, it is considerably difficult to automatically detect the density thereof.
Typically, two density detecting methods utilizing the optical means are known in the art. In one of the two methods the variations in volume of the developer are detected. In the other method the variations in optical reflection or transmission of the developer are detected. In these methods, it is necessary to select developers having characteristics which are suitable for the detection methods. However, the developers thus selected are not always suitable for the object and performance of the copying machine.
Another density detecting method utilizing optical detecting means has been disclosed by Japanese Patent Application Publication No. 46095/1977. In this method, the floating toner of the developer being circulated is allowed to stick to a probe (or a pulley) provided for detecting the density, so that the variation of light quantity attributing to the toner stuck on the probe is detected to obtain the density. However, it has been found as a result of the experiments that the method involves the following facts:
It is considered that one motive power causing the amount of toner proportional to the developer density to stick to the conductive probe attributes to the amount of charge in the toner. If the toner is caused to attach to the probe merely because of the physical property of the toner, then the toner should be similarly attached thereto even if the carrier, the other component of the developer, is changed. The carriers are classified according to the configuration, grain size, electric current value, etc. The curve A in FIG. 1 indicates the relation between the toner density and the amount of attached toner of a developer which includes a typical non-spherical carrier 150-250 meshes in grain size and 73.5 .mu.A in current value. As the number of chances of rubbing the toner against the carrier is increased, the amount of charges in the toner is increased. The curves A.sub.O and A.sub.N in FIG. 2 indicate the variations of the amount of charges when the use of the above-described developer is started and the variations of the amount of charges after the developer has been used for producing N sheets of copies. In this case, as the developer density is increased, the amount of charges is decreased and the force of combining the toner with the carrier is decreased. Therefore, it can be explained that the toner can readily attach to a member such as the probe when the developer density is increased.
The curve B in FIG. 1 and the curves B.sub.O and B.sub.N in FIG. 2 indicates the relations between the amount of attached toner and the amount of charges in toner obtained when a developer containing the same toner as that in the case of the curve A described above and spherical carrier 100-250 meshes in grain size and 42.8 .mu.A in current value is employed. In the case of this developer, the amount of charges in the toner is smaller, as a whole, than that in the case of the aforementioned developer and is inversely proportional to the density (in FIG. 2). However, the amount of toner attached to the probe is saturated to the extent that it is substantially difficult to distinguish, with a density of more than 2% (in FIG. 1). More specifically, the amount of charges in the toner when the density of the developer B containing the spherical carrier is at 2% corresponds substantially to that when the density of the developer A containing the non-spherical carrier is at 4%. This coincides with the detector output levels corresponding to the densities 2% and 4% in the curves A and B in FIG. 1. That is, when the toner charge amount is smaller than a certain value, then the amount of toner attached to the probe is saturated, as a result of which the density detection is impossible. In other words, depending on the kind of carrier to be combined with the toner, the toner affects the amount of charges therein. Thus, it can be understood that, in the density detecting method, the lower limit of the toner charge amount of the developer used is high, and the range of use thereof is limited by the characteristic thereof. The toner attaching conditions with respect to the environmental humidity are greatly affected by the humidity characteristic of the toner and by the constructional limitation that the probe must be disposed outside the developer circulating path. Thus, the conditions of use will be further limited.
In the above-described method, the detection may be performed substantially in proportion to the actual density of the developer if the developer and the environmental conditions are suitable. However, as the environmental humidity is changed, the toner attaching condition is greatly affected by the variation in characteristic of the toner with respect to humidity and by the fact that the probe is provided outside the developer circulating path. With a humidity higher than a certain value, the density detecting device according to the method is useless. Similarly as in the above-described two detecting methods, it has been found that the kinds of developers employable for density detection in this method are limited. Furthermore, not only in the described method but also in most of the conventional methods, it is necessary to additionally provide a special device having considerably intricate mechanisms for density detection, with the result that the manufacturing cost is increased. Thus, it has become imperative to improve conventional methods.